In Xanathar's Guide to Everything near the bottom of the first column on page 82 is a table for Herbalism Kit Activities. One of the Activities is named "Identify poison" and requires a DC 20 to be successful. Since a DC 20 check is a bit high for identifying a poisonous plant (assuming the user has proficiency), I'm trying to determine for what types of poison this Activity is applicable.
My impression is that the DC 20 check to "Identify poison" may have been in the context of determining whether a Potion is a Potion of Poison. But in the case where it is implied that a user's experience making antitoxin (for those with proficiency) has given them insight on poisons, would the activity apply to all poisons, or just plant poisons? (Some poisons are derived from creatures or magic.)
(See this related question that clarifies that the intended use is indeed to identify poison, and not a potion, because except for a potion of poison, those can be identified by taste).
Question: Does the Herbalism Kit Activity "Identify poison" apply to all poisons or only plant poisons, and can it be used on a Potion to determine if it is a Potion of Poison?
Here is my reasoning for use on Potions of Poison:
None of the Herbalism Kit Skills indicate a thorough knowledge of poisons in general, but (for those with proficiency) the Special Use for the Herbalism Kit "Identify Plants." states:
Identify Plants. You can identify most plants with a quick inspection of their appearance and smell.
This indicates that identifying a poisonous plant is trivial in most cases (when the user has proficiency). This also suggests that the Activity "Identify poison" with a DC 20 may not be for identifying poisonous plants.
However, the Arcana skill description of the Herbalism Kit states:
Arcana. Your knowledge of the nature and uses of herbs can add insight to your magical studies that deal with plants and your attempts to identify potions.
Since identifying potions in 5E is trivial, it seems unusual to roll an Arcana check to determine what kind of potion it is. However, in the context of determining if it is genuine or a Potion of Poison, a DC 20 appears to be a reasonable check.
Best Answer
It works on any normal poison
Identify plants does not require a skill check, and includes identifying poison plants. These are not poisons, processing is needed to extract the poison from the plant, and there is a special tool proficiency for creating actual poisons, the Poisoner's Kit1 .
Identify Poison from the activity table is not for identifying poison plants, it is for identifying outright poisons, including basic poison.
The DC of 20 is not unduely high. The main purview of the Herbalism Kit proficiency is to deal with medicinal plants to help with healing or allay poisoning. This requires some level of understanding of poisons, and that is expressed in the ability to identify them, but it does not make you an expert in poisons. Poisons in general are difficult to deal with in the game, so they require the special Poisoner's Kit proficiency. Also narratively, it is much easier to recognize the form and smell of a plant, than the nature of a processed (and illegal) substance2.
Potion of Poison
It's unlikely Identify Poison would identify a potion of poison, as in the potion's case, the nature of the potion is magically masked from you. Its description states that you need an identify spell to reveal its true nature. It's likely that detect poison and disease will also work, but mundane means likely will not. This may be up to your DM to decide.
1 Weirdly, called Poisoner's Tools in Xanathar's, I guess the editing there was not the best.
2 Thanks to Matthieu for pointing this out.